What is the electric field 4cm from a uniformly charged plate?

[roflcopter]

Homework Statement

Charge is distributed uniformly on the surface of a large flat plate. The electric field 2cm from the plate is 30 N/C. The electric field 4cm from the plate is:

A. 120 N/C
B. 80 N/C
C. 30 N/C
D. 15 N/C
E. 7.5 N/C

Homework Equations

E=kq/r^2 (possibly)

The Attempt at a Solution

Well, the answer I got was E, 7.5 N/C. I got that by using the above equation, plugging in 30 for E and 2 for r and solving for q. Then, I used that same value of q and the new r to solve for E and got 7.5 N/C. Supposedly the correct answer is C, 30 N/C. I don't understand how they got that though. Why is the E field still the same? Shouldn't it be decreasing with the greater distance?

Anyways, I think I might be wrong because I remembered afterwards that E=kq/r^2 is the E field due to a point charge and not a plate...maybe that's why I got the wrong answer?

 

[LowlyPion]

... I remembered afterwards that E=kq/r^2 is the E field due to a point charge and not a plate...maybe that's why I got the wrong answer?

I think you are on to something.

What is the field of a sheet of charge?

 

[roflcopter]

I think you are on to something.

What is the field of a sheet of charge?

Oh. I believe it's in here: http://230nsc1.phy-astr.gsu.edu/hbase/electric/elesht.html

So the eq looks like E=charge density/2*epsilon. So this is independent of distance. I see.

 

[LowlyPion]

Oh. I believe it's in here: http://230nsc1.phy-astr.gsu.edu/hbase/electric/elesht.html

So the eq looks like E=charge density/2*epsilon. So this is independent of distance. I see.

So ... then you have your answer.

 

[roflcopter]

Yep. Thanks for the help!

 

[LowlyPion]

Actually you solved it yourself.

The best outcome of all.